Indirect calorimetry and stable isotope technique will be used to investigate the progressive (temporal) alterations in lipid, glucose and protein metabolism during short-term fasting in young women and men, who meet the military body-weight and percent body-fat standards. Moreover we will test the hypothesis that a prior high intake of protein (2.5 g/kg-1/day-1) modifies the metablolic response to a short-term (3 day) fast, with preferential sparing of body protein in both women and men.